agreement with avoir

Question

Hi, I have a question re the following answer:
J'ai encore des cadeaux à acheter. - Marie les a déjà tous achetés
Why is the "achetés" plural in this case. As it is not conjugated with être in passé composé my understanding is that it would not agree?

Accepted Answer

Marie les (=des cadeaux) a achetés. -- Marie bought them (=some presents).
As you can see, the pronoun les refers to des cadeaux and functions as the COD in the sentence. With the COD preceding the participle (achetés), the participle needs to match the COD in number and gender.
Here's a lesson on this: https://french.kwiziq.com/revision/grammar/special-cases-when-the-past-participle-agrees-in-number-and-gender-when-used-with-avoir-in-le-passe-compose-conversational-past